Many Americans die each year of congestive heart failure. Heart failure may occur from a variety of causes, including cardiomyopathy, myocardial ischemia, congenital heart disease, and valvular heart disease, resulting in cardiac cell death and myocardial dysfunction. As cardiomyocytes are not replaced in adult myocardial tissue, physiologic demands on the existing, healthy, cardiomyocytes leads to their hypertrophy. Heart transplants have been the only recourse for patients in end-stage heart disease, however the United Network of Organ Sharing (UNOS), has reported that although more than 40,000 patients were waiting for heart transplants as of February 2000, only 2,345 people received a donated heart in 1998. Furthermore, heart transplants are complicated by the incompatibility between the transplanted donor tissue and the recipient's immune system, which requires life-long immunosuppression. Yet another drawback of heart transplants is their high cost.
An alternative approach to heart transplantation is to generate cardiomyocytes from stem cells in vitro that can be used in the treatment of heart failure, and other cardiac diseases characterized by myocardial cell death or dysfunction. This approach is based on the ability of stem cells to both self-renew and differentiate into one or more mature cell types, including cardiomyocytes. Stem cells may be obtained from an individual suffering from heart disease and then used to generate cardiomyocytes in vitro in order to repair the damaged myocardium. This approach avoids problems inherent with heart transplantation, such as lack of a suitable heart for transplant or immune rejection of a transplanted heart.
Embryonic stem (ES) cells, derived from the inner cell mass of the blastocyst, are the most primitive stem cell, as disclosed in WO 01/11011 A2. These cells have unlimited self-renewal capability, and because they can differentiate into several cell lineages and repopulate tissues upon transplantation, they have multipotent differentiative potential. However, protocols are not available for differentiating embryonic stem cells into beating cardiomyocytes.
Lineage specific stem cells, identified in most organ tissues, have less self-renewal capability than ES cells and their differentiative ability is limited to tissues of that lineage. Of the lineage specific stem cells, the hematopoietic stem cell (HSC), derived from bone marrow, blood, cord blood, fetal liver and yolk sack, is the best characterized. These cells are defined by the expression of cell surface markers, such as c-kit (c-kit+), and can terminally differentiate into all the hematopoietic cell types. HSC have been shown to contribute to the formation of functional cardiac tissue in vivo (Jackson et al, J. Clin. Invest., 107:1395–1402, 2001). Mesenchymal stein cells (MSC) are pluripotent progenitor cells derived from tissues of mesodermal origin (U.S. Pat. No. 5,486,359). These cells are most often obtained from bone marrow, although they can be obtained from other sources, such as blood or dermis. These cells have been shown to differentiate to form muscle, bone, cartilage, fat, marrow stroma and tendon, but have not been shown to differentiate into cardiomyocytes. In addition, progenitor cells have been identified in skeletal muscle, termed satellite cells (Cornelison and Wold, Dev. Biol., 191:270–283, 1997). These cells are characterized by the expression of the cell surface marker c-met (c-met+) in both its quiescent and activated states. When activated these cells re-enter the cell cycle, express myogenic regulatory factors, and differentiate into myoblasts.
However, despite the existence of a variety of stem cells, there is currently no pure population of stem cells that can be induced under defined conditions to differentiate into spontaneously beating cardiomyocytes in vitro. Thus, there remains a need in the art for isolated populations of stem cells which can be induced to differentiate into cardiomyocytes.